1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention is a papermaker's fabric for use in the press section of the papermachine, such a fabric being commonly referred to as a press fabric.
2. Description of the Prior Art
During the papermaking process, a fibrous web is formed by depositing a fibrous slurry on a forming fabric in the forming section of a papermachine. A large amount of water is drained from the slurry during this process, leaving the fibrous web on the surface of the forming fabric.
The newly formed web proceeds from the forming section to a press section, which includes a series of press nips. The fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the fibrous web is subjected to compressive forces which squeeze water therefrom. The water is accepted by the press fabric or fabrics and, ideally, does not return to the web.
The web finally proceeds to a drying section, which includes at least one series of rotatable dryer drums or cylinders, heated from within by steam. The web is directed in a sinuous path sequentially around each in the series of drums by a dryer fabric, which holds the web closely against the surfaces of the drums. The heated drums reduce the water content of the web to a desirable level through evaporation.
The press fabrics used to clothe the press section are crucial components in the paper manufacturing process. One of their functions is to support and to carry the paper product being manufactured through the press nips. In this respect, the fabric serves as a conveyor belt during the manufacturing process.
The press fabrics also take part in the finishing of the surface of the paper sheet. That is, the surface of the press fabric is designed to be smooth and uniformly resilient, so that, in the course of passing through the press nips, a smooth, mark-free surface is imparted to the paper.
Perhaps most importantly, the press fabrics accept the large quantities of water extracted from the wet paper in the press nip. In order to fill this function, there literally must be somewhere in the fabric for the water to go, and the fabric must have an adequate permeability to water for its entire useful life.
Contemporary press fabrics are available in a wide variety of styles designed to meet the requirements of the papermachines on which they are installed for the paper grades being manufactured. Generally, they comprise a woven base fabric into which has been needled a batt of fine, nonwoven fibrous material. The base fabrics may be woven from monofilament, plied monofilament, multifilament or plied multifilament yarns, and may be single-layered, multi-layered, or multi-plied.
The yarns are typically extruded from any one of the synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinarily skill in the papermachine clothing arts. Monofilament yarns may be used either singly or, as implied above, in a plied bundle comprising three or more monofilaments and having a minimal twist to hold the monofilaments together. Individual monofilaments typically have diameters over 0.18 mm.
Multifilament yarns may also be used singly or in a plied bundle of three or more individual multifilament yarns. A single multifilament yarn is composed of a great number of individual filaments of small diameter, typically less than 0.04 mm. The filaments are twisted together to some degree to impart integrity to the multifilament yarn. In a plied multifilament yarn, three or more multifilament yarns are twisted minimally about one another to hold the multifilaments together.
Base fabrics woven from monofilament or plied monofilament are open, providing ample void volume for the temporary storage of water pressed from the paper sheet, and relatively incompressible because of the stiffness of the monofilament. On the other hand, those woven from multifilament or plied multifilament are compressible, because of the thinness of the individual filaments making up the multifilament yarns, and tend to become compacted during use on the papermachine, with the result that they lose openness and void volume.
In each case, the yarns themselves to some extent block the passage of water through the base fabric. Where monofilaments are used, the blockage occurs because of the relative thickness of the yarns. Where multifilaments are used, the blockage occurs because the individual filaments in the multifilament yarns are so thin that they clump together with little space therebetween for the passage of water. In each case, water must pass around the yarns, rather than through them, to drain or pass through the base fabric. The result may be marking of the paper sheet caused by an uneven pressure distribution.
The present invention provides a solution to these problems in the press fabrics of the prior art.